What is the “Pin 1 problem” and how do I avoid it?
This is a special case of “ground loop” or shared path coupling. Recently this has been discussed in great detail and clarity by a group led by the consultant Neil Muncy of Toronto. Suppose you have a mixer, whose balanced output is connected to an amplifier’s balanced input through a correctly wired cable. Both units are powered from the AC mains and one or both have some small amount of AC leakage current that travels to ground through all available ground paths — including the shield of the cable that connects the two units. So far so good, no harm done because the circuit is balanced and any common mode voltage from current flowing through the shield will be canceled by the amplifier input. However… a small part of this leakage current also travels through the shield of the wire going from the back panel XLR connector to the PC board, through some “ground” traces on the PC board, and back out through the power line ground cable. No problem so far, except that some gain stage on
